study on the limit load of large diameter natural gas

ScientificJournalofTechnologyVolume1Issue02,2019

ISSN:2688‐8645

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StudyontheLimitLoadofLargeDiameterNaturalGasPipelinewithInternalandExternalDefects

FeihongQina,LeiYangb,KaidieYangc,YiLidandYuchuanGuoe

Schoolofpetroleumengineering,SouthwestPetroleumUniversity,Chengdu610500,Chinaa528960108@qq.com,b1358480650@qq.com,c2824146036@qq.com

d1125113622@qq.com,e1971945328@qq.com

AbstractInthispaper,theD1016×18.4X80pipelineinarivercrossingsectionofMyanmarChinanatural gas pipeline project is taken as the research object, and the plastic failurecriterion and the double elastic slope criterion are selected as the determinationmethodsof theultimate internalpressure load.Basedon theABAQUS finiteelementanalysis,theeffectsofthedepth,lengthandwidthofinternalandexternaldefectsontheultimateloadofthepipelinearestudiedrespectively.

Keywords

Large Diameter Pipeline; Ultimate Load; X80 Pipeline Steel; Internal And ExternalDefects;Interference.

1. Introduction

Oil and natural gas promote the development of the world economy, which is of greatsignificance to the national economy and national development. At present, the pace ofdevelopmentofChina'soilandnaturalgasindustryisspeedingup,andtheconstructionofoilandgaspipelineshasenteredahigh‐speeddevelopmentperiod.Therearemanyreasonsforpipeline corrosion, moreover the mechanism is complex and the location of corrosion israndom.Theappearanceofcorrosiondefectswillaffectthefailureformandultimatepressureofpipeline.Thelong‐distanceoilandgaspipelinepassesthroughawideareawheretheterrainis complex. Furthermore, there are different soil properties, diverse pipeline structure andvarioustransmissionmediumproperties,thusitiseasytocausealargenumberofinternalandexternaldefectsofthelong‐distancepipeline.Thepipelinewithinnerandouterwalldefectsdoes not necessarily lose the bearing capacity. Repair and replace the pipeline when thepipeline does not completely lose the bearing capacity,which will force to stop pipelinetransmission,causingunnecessaryeconomiclossestoacertainextent.Basedonthefiniteelementmethod,X.Lietal.[1](2016)proposedanevaluationmethodforthe interference between adjacent corrosion, which has better accuracy and applicability,especiallyfortheaxiallongcorrosiondefects.S.s.al‐owaisietal.[2](2016)usedfiniteelementsoftwaretoanalyzeX60withadjacentdefectsofdifferentshapes.Itwasfoundthattheadjacentdefectsoftwoshapes(ellipseandrectangle)willinteractwhentheaxialdistanceisnotmorethan3timesofthewallthicknessorthecircumferentialdistanceisnotmorethan0.5timesofthewallthickness.InGB/T19624safetyassessmentofpressurevesselswithdefectsinservice(2004) [3], considering themutual influence ofmultiple cracks, a crackmergingmethod isproposedaimingatmultipleadjacentcracks,inwhichtheinternalandexternalcrackdefectsare treated as penetrating crack defects, but this method does not propose the treatmentmethodofinternalandexternalvolumecrackdefects.Tosumup,therearefewresearchresultsonthelimitloadofdoubledefectpipelineatpresent,except for the research of crack defects on the inner and outer walls of the pipeline.

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Furthermoretherearefewresearchonthelimitloadofthepipelinewithinternalandexternalvolumedefects.Atpresent,theresearchonthelimitloadofthepipelinewithdoubledefectsismainlyaimedatthesamesurfacedefects,whereasthereisnowaytoavoidtheexistenceandinteraction of internal and external wall defects in actual working condition. Adopting thecurrentpipelinesafetyevaluationsystemtodeterminethepipelinewithinternalandexternaldefects will result in relatively conservative judgment results, thus increasing the pipelineoperationrisk.Therefore,itisnecessarytofurtherstudytheultimateloadofpipelineundertheinterferenceofinternalandexternaldoubledefects.

2. LiteratureReferences

Theprocessofpipelinematerialfailureisrelativelycomplex.Throughthecomparativeanalysisoftheexistingtheoryandmethodofpipelineultimateloadandpipelinefailurecriterion,thispaper finally selects the plastic failure criterion to analyze the ultimate bearing capacity ofpipeline,andchoosesthedoubleelasticslopemethodasthedeterminationcriterionofpipelineultimateload.

2.1. PlasticFailureCriterionInordertoobtaintheultimateloadwithengineeringpracticalsignificance,theplasticfailurecriterion is used to analyze the ultimate bearing capacity. The plastic failure criterion isexpressedasfollows:

00

2ln

3L f

i

RP

R (2‐1)

Inwhich

0LP ——Plasticlimitinternalpressureofflawlesspipelineunderinternalpressure,MPa;

0R ——Outerradiusofpipe,mm;

iR ——Innerradiusofpipe,mm.

The X80 steel studied in this paper belongs to high‐strength steel grade, which has greattoughness.Inordertoexertthebearingcapacityofthematerialandobtainthelimitloadwithengineeringpracticalsignificance,thelocalandcontrollableplasticdeformationofthepipelineisallowed.Inconclusion,thefailurestateofthepipelinewithinternalandexternaldefectsisanalyzedbyusingtheplasticfailurecriterioninthispaper.

2.2. LimitLoadDeterminationCriteriaAccordingtothedifferentcriteriaofsignificantplasticflowwhenthesteelreachestheplasticlimit,therearemanycriteriatodeterminetheultimateloadinengineering.Inthispaper,thedoubleelasticslopecriterionisusedasthecriteriatodeterminetheultimateload,asshowninFigure1,makeaninternalpressure‐straincurve,andmakeastraightlinethroughtheorigin,sothatitsslopeistwicetheslopeoftheelasticsectionoftheloaddeformationcurve.Theloadvalue corresponding to the intersectionof the straight lineand the internalpressure‐straincurveisthelimitload.

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tanψ=2tanθ

Limit load line

Ultimate load point

Ultimate test load

Regression line

Maximum principal strain or displacement

ψ

θ

Loa

d

Figure1:schematicdiagramofdoubleelasticslopecriterion

ThedoubleelasticslopecriterionisamoderncriterionadoptedbyASMEBoilerandpressurevesselcode.IthasarelativelygoodapplicabilityandhighaccuracytothemainstreamsteelinChina.Therefore,thedoubleelasticslopecriterionisadoptedasthedeterminationcriterionofultimateloadinthispaper.

3. FiniteElementAnalysisofLimitLoadofInternalandExternalDefects

3.1. ModelEstablishmentandParameterSettingInthispaper,theparametersofarivercrossingsectionofMyanmarChinanaturalgaspipelineare used for modeling and finite element analysis. In engineering practice, the pipeline isaffectedbymanyfactors.Inordertoaccuratelyanalyzethepipeline,thefull‐sizeD1016×18.4steelpipemodelisusedinthispaper.According toSaintVenant's theorem[4], the lengthof thepipeaffectedby theendeffect iscalculatedasfollows:

10162.5 2.5 18.4 241.7

2L Rt mm (1)

Itshowsthatthelengthof241.7mmawayfromthetwoendswillbeaffectedbytheconstraintsofthetwoends.Thus,inordertofullyavoidtheinfluenceoftheendeffectofthepipesection,bettermodelingandanalysis,thelengthofthefiniteelementmodelinthispaperissetas4m,andthedefectsarelocatedinthemiddleofthemodel.ModelparametersareshowninTable1.

Table1:finiteelementmodelparametersofpipesection

Steels

Yieldstrength(M

Pa)

Ultimatetensile

strength(MPa)

Youngmodulus(G

Pa)

Poisson'sratio

Length(m)

Externaldiameter(m

m)

Wallthickness(m

m)

API5L

SPECX80

555 625 210 0.3 4 1016 18.4

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Themodelofpipesectionisformedbydrawing,thetypeofdefectissetasrectangularvolumedefect, and the corrosion rectangular pit is formed by rotating cutting. Besides, themodeladopts C3D8R eight nodes linear hexahedron reduction integral element. In addition, thestructuredgriddivisiontechnologyisusedtofinallyformthegrid.ThegriddivisionresultsareshowninFigure1.

Figure2:schematicdiagramofgriddivision

3.2. AnalysisoftheInfluenceofInternalandExternalDefectParametersontheUltimateLoad

3.2.1. AnalysisoftheInfluenceoftheDepthofInternalandExternalDefectsontheUltimateLoadofPipeline

Inthissection,theinfluenceofdifferentdefectdepthontheultimateloadwillbeanalyzed.Inordertofacilitatetheanalysisandsummaryofthelaw,thedefectdepthwillbedimensionlesstreated,anda/t=0.1willbeusedasthedefectfoundationdepthofthestudy.Whenstudyingtheinfluenceofthedepthofinternalandexternaldefectsontheultimateloadofpipeline,setthelengthofinternalandexternaldefects /b Rt =1,widthc=20mm,andthedepthaofinternalandexternaldefectsarea/t=0.1,0.15,0.2,0.25and0.3respectively.Thereare25typesofdefectsintotal.AsshowninFigure2,basedonfivekindsofpipeaxialspacingofL=50mm,150mm,250mm,350mmand450mm,125modelswithexternaldefectsa1/tandinternaldefectsa2/tof0.1,0.15,0.2,0.25and0.3,wererespectivelyestablished.

(a)a1/t=0.1(b)a1/t=0.15(c)a1/t=0.2(d)a1/t=0.25(e)a1/t=0.3

Figure3:schematicdiagramofexternaldefectmodelbasedondepthchange

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Theabove125kindsofpipelinemodelsareimportedintoABAQUSforfiniteelementanalysis,andthecorrespondinginternalpressure‐straincurveisobtained.Further,thelimitloadcanbeobtainedbyintroducingitintoOriginsoftwarewiththeuseofthedoubleelasticslopemethod.

L=50mm

Figure4:3Drainbowmapofinnerdefectdepthouterdefectdepthultimateload

0.10 0.15 0.20 0.25 0.300.10

0.15

0.20

0.25

0.30

Dep

th o

f ex

tern

al d

efect

Depth of internal defect

13.18

14.30

15.42

16.54

17.66

18.78

19.90

21.02

22.14

L=50mm

Figure5:contoursectionofinternaldefectdepthexternaldefectdepthultimateload

ItcanbeseenfromFigure3andFigure4thatwhenL=50mm,withtheincreaseofthedepthofinternalandexternaldefects,thelimitloadvalueofpipelinedecreasesfrom22.13MPato13.19MPa,withatotaldecreaseof8.94MPaandthedecreaseof40.40%.Theaboveanalysisshowsthatwhentheaxialdistancebetweentheinnerandouterdefectsissmall,theinterferenceeffectofthedoubledefectsisnoticeablystrengthened,whichleadstothedeclineoftheultimateloadofthepipeline.ItcanbeseenfromFigure4thattheultimateloadofpipelinedecreasesrapidlywiththeincreaseofdefectdepth,andtherateofchangeisincreasing,whichfullyshowsthattheinfluenceofdefectdepthontheultimateloadofpipelineissignificant.

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3.2.2. AnalysisoftheInfluenceoftheLengthofInternalandExternalDefectsontheUltimateLoadofPipeline

Inthissection,theinfluenceofdifferentdefectlengthsontheultimateloadwillbeanalyzed.Inordertofacilitatetheanalysisandsummaryofthelaw,thedefectlengthwillbedimensionlesstreatedand /b Rt =0.5willbeusedasthebasicdefectlengthofthestudy.Whenstudyingtheinfluenceofthelengthofinternalandexternaldefectsontheultimateloadofthepipeline,setthedepthofinternalandexternaldefectsa/t=0.25,a=4.6mm,widthc=20mm,andthelengthbofinternalandexternaldefectsare0.5 Rt , Rt ,1.5 Rt ,2 Rt ,2.5 Rt ,respectively.Thereare25typesofdefectsintotal.AsshowninFigure5,basedonfivekindsofpipeaxialspacingofL=50mm,150mm,250mm,350mmand450mm, the lengthofexternaldefectb1andinternaldefectb2 as0.5 Rt , Rt ,1.5 Rt ,2 Rt , 2.5 Rt are respectivelyestablished,andthereare125pipemodelsintotal.

(a)b1=0.5 Rt (b)b1= Rt (c)b1=1.5 Rt (d)b1=2 Rt (e)b1=2.5 Rt

Figure6:schematicdiagramofexternaldefectmodelbasedonlengthchange

Theabove125kindsofpipelinemodelsareimportedintoABAQUSforfiniteelementanalysis,andthecorrespondinginternalpressure‐straincurveisobtained.Further,thelimitloadcanbeobtainedbyintroducingitintoOriginsoftwarewiththeuseofthedoubleelasticslopemethod.

L=50mm

Figure7:3Drainbowmapofinnerdefectdepthouterdefectdepthultimateload

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L=50mm

Figure8:contoursectionofinternaldefectdepthexternaldefectdepthultimateload

ItcanbeseenfromFigure6thatwhenL=50mm,withtheincreaseofthelengthofinternalandexternaldefects,theultimateloadvalueofthepipelinedecreasesfrom19.41MPato14.83MPa,withatotalof4.58MPaandthedecreaseof23.60%.Thus,theultimateloadofpipelinewilldecreasewiththe increaseofdefect length,butthechangeratewillbesmallerandsmaller.Besides,asshowninFigure7,thecontourdistributionbecomessparserwiththeincreaseofthelength,indicatingthatwhenthedefectlengthislonger,itsinfluenceontheultimateloadofthepipelinedecreasesgradually.3.2.3. AnalysisoftheInfluenceoftheWidthofInternalandExternalDefectsonthe

UltimateLoadofPipelineThissectionwillanalyzetheinfluenceofdifferentdefectwidthontheultimateload.Inordertofacilitatetheanalysisandsummaryofthelaw,widthc=20mmisusedasthewidthofdefectfoundation.Whenstudyingtheinfluenceofthewidthofinternalandexternaldefectsontheultimateloadofthepipeline,setthedepthofinternalandexternaldefectsa/t=0.1,a=1.84mm,lengthb/ Rt=1,b=96.68mm,andthewidthcofinternalandexternaldefectsarerespectively20mm,50mm,100mmand200mm.Thereare16typesofdefectsintotal.AsshowninFigure8,themodelswiththewidthofexternaldefectc1andinternaldefectc2of20mm,50mm,100mmand200mmarerespectivelyestablishedbyusingtheaxialspacingofL=50mmand150mm.Thereare32pipelinemodelsintotal.

(a)c1=20mm(b)c1=50mm(c)c1=100mm(d)c1=200mm

Figure9:schematicdiagramofexternaldefectmodelbasedonwidthchange

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Theabove32kindsofpipelinemodelsareimportedintoABAQUSforfiniteelementanalysis,andthecorrespondinginternalpressure‐straincurveisobtained.Further,thelimitloadcanbeobtainedbyintroducingitintoOriginsoftwarewiththeuseofthedoubleelasticslopemethod.ItcanbeseenfromFigure9thatwhenL=50mm,withtheincreaseofthewidthofinternalandexternaldefects,thelimitloadvalueofpipelinedecreasesfrom22.13MPato21.48MPa,withonlyatotaldecreaseof0.65MPaandadecreaseof2.94%.Inaddition,withtheincreaseofthewidthofinternalandexternaldefects,thelimitloadofthepipelinedecreasesslightly,whiletheoverallchangeisnotsignificant.Furthermore,itcanbeseenfromFigure10thatmostareasoftheultimateloadareinastateofvery low rate of change, indicating that the influence of thewidth of internal and externaldefectsontheultimateloadofthepipelineisinsignificant,whichcanbeignoredinengineering.

L=50mm

Figure10:3Drainbowmapofinnerdefectdepthouterdefectdepthultimateload

L=50mm

Figure11:contoursectionofinternaldefectwidthexternaldefectwidthultimateload

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4. Conclusion

In this paper, theD1016×18.4X80pipeline of a river crossing section ofMyanmarChinanatural gas pipeline project is used as the modeling basis, meanwhile the ABAQUS finiteelementsimulationisusedtoresearchontheinfluenceofinternalandexternaldefectsontheultimateloadofthepipelineundertheinterference.Besides,theinfluenceofthedepth,lengthand width of internal and external double defects on the ultimate load of the pipeline isanalyzedanddiscussedrespectively.Theconclusionisasfollows:Theultimateloadofpipelinedecreasessignificantlywiththeincreaseofthedepthofinternalandexternaldefects,andtherateofchangeisincreasing.Thus,thedepthofdefectsisthemainfactoraffectingtheultimateloadofpipeline.While,theultimateloadofpipelinedecreaseswiththe increaseof the lengthof internal and external defects, but the rate of change is gettingsmallerandsmaller,whichindicatesthatthelengthofdefectshasanimpactontheultimateloadofpipelinetosomeextent.Furthermore,whenthelengthofdefectsislongerthan2.5 Rt ,theinfluenceoflengthonthepipelinecanbeignored.Moreover,comparedwiththedepth,thedefectlengthisthesecondaryinfluencefactoroftheultimateloadofthepipeline;atlast,theultimateloadofthepipelineisalmostconstantwiththeincreaseofthewidthoftheinternalandexternaldefects,indicatingthatitsinfluencecanbeignored.

Acknowledgements

SupportedbytheOpenExperimentFundofSouthwestPetroleumUniversity.

References

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